The present invention relates to a method for reducing friction phenomena between a flowing fluid and the wall of a pipe in which it is carried.
It is applicable to transportation of gas under pressure, particularly that of natural gas under pressure, by reducing the pressure losses resulting from friction phenomena.
It relates in particular to a method for transporting natural gas under pressure in the presence of a liquid fraction, whereby the flow of the gas plus the liquid fraction is annular over at least part of the length of the pipeline.
Transportation of natural gas under pressure represents an important economic issue and intercontinental gas pipelines require substantial investment outlays. The gas is generally transported over long distances, with the transportation pressure being approximately 70 bars in the case of a land line but as high as approximately 160 bars in the case of a sea line. In the majority of cases, the natural gas transported contains at least 90% methane.
The pressure losses resulting from friction between the gas in turbulent flow and the inside wall of the pipeline are large, due in particular to the long distances, and contribute to increasing the cost of production. Hence it is necessary to reduce these pressure losses.
One method for offsetting pressure losses resulting from these friction phenomena consists of installing recompression stations regularly spaced along the pipeline and regulated to offset pressure losses. Such an arrangement considerably increases the necessary investment outlay and entails high energy consumption and operating costs.
Moreover, for sea lines, the recompression process during transportation presents implementation difficulties.
The prior art describes the use of additives such as polymers added to the fluid to be carried in specific proportions to reduce friction phenomena.
The method described in U.S. Pat. No. 4,958,653 enables the friction-reducing additives added to a pipeline to be proportioned in such a way that the gas flowrate is maximized for fixed pressure loss conditions.
The principal teaching of this reference is based on a method principally involving the following steps: a gas is introduced into a line under pressure, a quantity of additives is injected at a first point in the line, the gas flowrate is checked at a second point in the line, and the quantity of additives injected at the first point is adjusted until the gas flowrate measured at the second point reaches a maximum value.
The additive also fills in the unevennesses in the inside wall of the pipeline and decreases friction phenomena by minimizing protuberances.
However, such a method does not enable the pressure loses to be minimized for a fixed gas flowrate.
To transport a viscous liquid such as crude petroleum, a method has been described consisting of adding a less-viscous liquid, for example water, which flows with the higher-viscosity liquid and thus reduces wall friction.
It has been discovered that, in the case of a gas flowing under pressure, it is possible to reduce the pressure loss by injecting a liquid that is more viscous than the gas transported, meeting conditions that produce annular flow, as described in French Application EN. 96/08,914.